The present invention relates to a method for operating an exhaust gas aftertreatment system with at least one first SCR device and at least one second SCR device and a dosing device for reactant for the SCR devices disposed upstream of the first SCR device in the exhaust gas flow direction. Moreover, the present invention relates to a computer program and a computer program product with program code that are suitable for implementing said method.
In order to satisfy increasingly stringent emission legislations, especially for motor vehicles, it is necessary to reduce the content of nitrogen oxides (NOx) in the exhaust gas of internal combustion engines, in particular for diesel engines. For this purpose, SCR catalyzers (Selective Catalytic Reduction) are known, which are disposed in the exhaust gas region of an internal combustion machine, wherein the SCR catalyzers reduce the nitrogen oxide contained in the exhaust gas of the internal combustion machine to nitrogen in the presence of a reducing agent. For the process of the reaction, ammonia (NH3) is required as a reducing agent or reactant, which is admixed with the exhaust gas. For the provision of ammonia an aqueous urea solution is normally used, which is introduced into the exhaust system upstream of the SCR catalyzer using a dosing device. NH3 separates from said solution and can act as a reducing agent in the SCR catalyzer. In order to achieve high conversion rates in the SCR catalyzer during the reduction of the nitrogen oxides, the SCR catalyzer must be operated such that it is continually filled up to a certain level with the reducing agent ammonia.
Systems are already known for achieving higher conversion rates of nitrogen oxide reduction in the exhaust system, which use two separate SCR catalyzers. So-called SCRF systems are also known, which comprise a particle filter that is coated with SCR material and a downstream conventional SCR catalyzer. Such systems with two SCR devices can be controlled in a conventional manner with duplicated software for process control purposes, wherein the variables for the process control for each SCR device can be obtained from known catalyzer models. Models of SCR catalyzers that are known from the literature can be implemented in the control devices of modern motor vehicles and represent both the NOx conversion of the SCR catalyzer and also the NH3 slip. As a result of the duplicated software or the duplicated modeling of the SCR devices provided in the system, i.e. in particular two SCR catalyzers connected in series, the process control for said system is relatively complex and also unreliable. In contrast, the object of the present invention is to improve and optimize the process control for two SCR devices connected in series.
This object is achieved by a method for operating an exhaust gas aftertreatment system with at least one first and at least one second SCR device according to embodiments of the invention. Preferred embodiments of said method and a corresponding computer program and a corresponding computer program product for implementing the method are apparent from the claims and description provided herein.
The method according to the invention is based on an exhaust gas aftertreatment system, which comprises at least one first SCR device and at least one second SCR device. These can be similar or even different types of SCR devices. For example, the first SCR device can be a particle filter with SCR coating (SCRF=SCR on Filter). The second SCR device can be a normal SCR catalyzer. Furthermore, it is provided that the second SCR device is supplied with reactant that escapes from the first SCR device. The second SCR device thus receives the reducing agent from the NH3 slip of the first SCR device, e.g. from the particle filter with SCR coating. The first SCR device is supplied with reactant by a dosing device, wherein the dosing device is disposed upstream of the first SCR device looking in the exhaust gas flow direction. Unconverted reactant in the first SCR device passes through the first SCR device and is available to the second SCR device for reduction of the nitrogen oxides. The core of the invention is that an overall optimization for NOx and NH3 is achieved using an analytical overall SCR model. In contrast to conventional process control for systems with two SCR devices, the SCR devices are not optimized separately but as an entire system. For this purpose, according to the invention a desired overall efficiency ηDes is specified for the two SCR devices. Using modeling of the exhaust gas aftertreatment system, depending on the specified overall efficiency ηDes a target value ⊖1,Des is determined, wherein ⊖1,Des represents the degree of charge of the first SCR device with reactant. Finally, the dosing of the reactant that is necessary to achieve the target value ⊖1,Des is adjusted, wherein the dosing device disposed upstream of the first SCR device is controlled accordingly. By this method considerable improvements can be achieved in the application and in the quality of the process control for exhaust gas aftertreatment systems with at least one first and at least one second SCR device. In particular, the NH3 supply of the downstream SCR catalyzer or the downstream SCR device is improved, wherein at the same time the reducing agent slip (NH3 slip) downstream of the second SCR device remains under control. Thus a significantly higher NOx conversion can be achieved at the same time as low NH3 slip of the entire system.
The essential optimizing target of the method according to the invention is the conversion of NOx. If a target NOx conversion is specified, which in particular is necessary to achieve a specified emission target, then for a known NH3 charge of the downstream SCR device it can be determined by computer how much reducing agent must be provided in the upstream SCR device (e.g. SCRF) in order to achieve said target. Said level or degree of charge of the first SCR device can be relatively rapidly adjusted by dosing the reactant. The strategy according to the invention optimizes the NH3 slip of the entire system at the same time, because the NH3 slip arises because of overfilling of the second SCR device with reactant. The higher the level of charge of the downstream SCR device, the more NOx conversion can be carried out by the second SCR device and the lower is the required level or degree of charge in the first SCR device. According to the invention, the tendency to NH3 slip, i.e. the NH3 that leaves the second SCR device, is therefore implicitly minimized.
The method according to the invention is not restricted to the coupling of an SCR coated particle filter with an SCR catalyzer. The advantages of the method according to the invention can also be used in the same way for e.g. two SCR catalyzers connected in series.
The target overall efficiency ηDes or corresponding variables that represent said overall efficiency can be specified as a fixed application value or depending on different variables, e.g. depending on the working point and/or the temperature in particular in the exhaust gas aftertreatment system and/or the mass flow in the exhaust system and/or the driving speed and/or the already accumulated NOx emissions.
Different variables are included in the modeling of the exhaust gas aftertreatment system (deNOx system), in particular the volumetric flow of exhaust gas and/or the mass flow of exhaust gas and/or temperatures in the system and/or the level or degree of charge in at least one of the SCR devices and/or the ratio of NO2 to NOx. The input variables can be measured or simulated. Using modeling of the deNOx system the target degree of charge (target degree of coverage) ⊖1,Des for the first SCR device, at which the deNOx system has an overall efficiency of ηDes, can be calculated.
The variables that are included in the modeling can be actual values or applicable values. In particular, an actual value or an applicable value, in particular an average anticipated value, can be used for the value of the volumetric flow of exhaust gas and/or the mass flow of exhaust gas.
The calculation of ⊖1,Des can e.g. be carried out using the following formula:
      θ          1      ,      Des        =                              -                                    V              .                        1                                                k                          reac              ⁢                                                          ⁢              1                                ⁢                      exp            ⁡                          (                                                -                                      E                                          reac                      ⁢                                                                                          ⁢                      1                                                                      /                                  (                                      R                    ·                                          T                      1                                                        )                                            )                                ⁢                      A            1                              ⁢              ln        ⁡                  (                      1            -                          η              Des                                )                      -                            k                      reac            ⁢                                                  ⁢            2                          ⁢                  θ          2                ⁢                  exp          ⁡                      (                                          -                                  E                                      reac                    ⁢                                                                                  ⁢                    2                                                              /                              (                                  R                  ·                                      T                    2                                                  )                                      )                          ⁢                  A          2                ⁢                  T          1                                      k                      reac            ⁢                                                  ⁢            1                          ⁢                  exp          ⁡                      (                                          -                                  E                                      reac                    ⁢                                                                                  ⁢                    1                                                              /                              (                                  R                  ·                                      T                    1                                                  )                                      )                          ⁢                  A          1                ⁢                  T          2                    
Here index 1 refers to the variables in the first SCR device and index 2 to the variables in the second SCR device. {dot over (V)}1 refers to the volumetric flow, kreac exp(− . . . ) refers to an Arrhenius rule known from the literature for a catalyzed reaction running in the SCR device, e.g. a certain NOx reaction or even other reactions, e.g. such as NH3 oxidation, nitrous oxide formation, nitrate formation or other. The Arrhenius rule illustrated in the formula stands by way of example for a first order reaction in NOx, i.e. the reaction rate of NOx rreac in [mol/s] is proportional to rreac˜kreaccNOxexp(−E/(R·T)). Here cNOx refers to the concentration of NOx at the inlet of the respective catalyzer. Ereac,i refers to the activation energy for the NOx reaction in the ith catalyzer (i=1 or i=2). A refers to the catalytically active areas in the SCR devices and T the temperatures in the respective SCR devices.
It is possible to use the different reactions known from the literature for an SCR reaction, in particular a standard SCR reaction, a slow SCR reaction and/or a fast SCR reaction as a basis for the calculation according to the invention. With particular advantage it is possible to combine the different known reactions to form a representative reaction and thus to represent all possible conditions in the SCR devices or in the entire deNOx system. Depending on the complexity of the underlying model, it can be that the equations can no longer be solved analytically for ⊖. In this case it is expedient to solve the equations numerically in the control device.
After determining the target value ⊖1,Des, i.e. the target value for the degree of charge of the first SCR device for a given overall efficiency ηDes, the dosing of the reactant is adjusted accordingly. Advantageously, the dosing of the reactant can be carried out with a level regulator, which is present in many conventional systems as standard. Here the term level refers to the product of the degree of coverage or degree of charge and the maximum level of the SCR device with reducing agent. Normally here a P regulator for level adjustment is set to a pilot component.
Furthermore, advantageously the degree of charge of the second SCR device can be monitored. Here the dosing can be adjusted such that the second SCR device is not excessively charged with reactant, and therefore an excessively large NH3 slip is avoided.
Furthermore, it can be provided that combustion residues, i.e. in particular soot or ash, in the SCR devices and in particular in the first SCR device, e.g. in the SCRF, are taken into account and e.g. are included in the modeling of the deNOx system.
Finally, the invention includes a computer program that executes all steps of the described method if it is implemented on a computing device or a control device, and a computer program product with program code that is stored on a machine-readable medium for implementing the method according to the invention if the program is executed on a computing device or a control device. The implementation of the method according to the invention as a computer program or as a computer program product has the advantage that said program can easily be used even in existing motor vehicles, in order to be able to exploit the advantages in the process control of an exhaust gas aftertreatment system with two or more SCR devices.